Sum of Root To Leaf Binary Numbers

sum_root_to_leaf_binary_numbers.py

"""
Given a binary tree, each node has value 0 or 1.  Each root-to-leaf path represents a binary number starting with the
most significant bit.  For example, if the path is 0 -> 1 -> 1 -> 0 -> 1, then this could represent 01101 in binary,
which is 13.

For all leaves in the tree, consider the numbers represented by the path from the root to that leaf.

Return the sum of these numbers.



Example 1:
       1
     /   \
    0     1
  /  \   / \
 0   1  0   1

Input: [1,0,1,0,1,0,1]
Output: 22
Explanation: (100) + (101) + (110) + (111) = 4 + 5 + 6 + 7 = 22
"""


# Definition for a binary tree node.
class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution:
    def sumRootToLeaf(self, root: TreeNode) -> int:
        stack = [(root, "")]
        res = 0
        while stack:
            node, path = stack.pop()
            if not node.left and not node.right:
                path += str(node.val)
                res += int(path, 2)
            if node.left:
                stack.append((node.left, path + str(node.val)))
            if node.right:
                stack.append((node.right, path + str(node.val)))
        return res